Autoimmune diseases are characterized by immune responses that are directed against self antigens. These responses are maintained by the persistent activation of self-reactive T lymphocytes. T lymphocytes are specifically activated upon recognition of foreign and/or self antigens as a complex with self Major Histocompatibility Complex (MHC) gene products on the surface of antigen-presenting cells (APC).
Myasthenia gravis (MG) is an autoimmune disorder, the symptoms of which are caused by an antibody-mediated autoimmune attack on the acetylcholine receptor (AChR) of the post-synaptic membrane of the neuromuscular junction. This antibody attack results in loss of acetylcholine receptors and jeopardizes normal neuromuscular transmission, leading to episodic muscle weakness, chiefly in muscles innervated by cranial nerves, and to fatigability.
T lymphocytes are considered to play a central role in the autoreactive process, but the specific immunoregulatory mechanisms by which the T cells exert their regulatory role leading to the induction and various clinical manifestations of MG are poorly understood, and no specific cure is available for the treatment of myasthenic patients. Presently, treatment is with cholinesterase inhibitors, such as pyridostigmine and neostigmine, thymectomy, corticosteroids, and immuno-suppressive agents and plasmapheresis.
MG is a well defined autoimmune disorder mediated by antibodies specific to determinants of the AChR. Specific genes of the human MHC, the HLA system, were shown to be significantly associated with the disease. The high frequency of certain histocompatibility antigens (HLA-B8, DR3) in MG patients suggests a defect of immunoregulation that might be expressed on the level of T cells.
In previous studies we found that two peptides representing sequences of the human AChR .alpha.-subunit (p195-212 and p257-269) significantly stimulated peripheral blood lymphocytes (PBL) from MG patients in comparison to healthy controls (Brocke, S. et al. (1988), J. Clin. Invest. 82:1894-1900). In addition, a correlation was demonstrated between the HLA-DR types of the MG patients and their responses to these peptides. Thus, all patients that expressed HLA-DR3 responded to p257-269 and 83% of patients who expressed HLA-DR5 responded to p195-212.
Extension of this research using inbred mouse strains led to the identification of high, intermediate and low responder strains to the sequences p195-212 and p259-271 of the human AChR .alpha.-subunit. Furthermore, lymph node cells, from Torpedo-derived AChR immunized SJL and BALB/c mice, proliferated in response to p195-212 and p259-271, respectively, even better than to the immunizing antigen (Brocke, S. et al. (1990), Immunol. 69:495). These results indicate that peptides p195-212 and p259-271 are immunodominant murine T cell epitopes.
Long-term T cell lines and clones of C3H.SW origin specific to synthetic immunogenic peptides p195-212 and p259-271 were established in our laboratory and described by Brocke, S. et al. (1990a), Internat. Immunol. 2:735-742, herein incorporated by reference. Using these cell lines and clones, it is possible to characterize the T cell recognition process of myasthenic epitopes. Using this method, T cell lines and clones specific to p195-212 were established from lymph node cells of low (C3H.SW) and high (SJL) responder mouse strains, and T cell lines and clones specific to p259-271 were developed from lymph node cells of low (C3H.SW) and high (BALB/c) responder mouse strains.
It has recently been reported that when activated cells of these cell lines are inoculated into naive syngeneic mice, peptide-specific antibodies and antibodies to the murine acetylcholine receptor are detected. In addition, decremented compound muscle action potentials (CMAP) consistent with impairment of neuromuscular transmission are seen in the line-inoculated mice. Thus, this can serve as a marine model of MG-related autoimmune manifestations. See Kirshner et al., (1994), Cell. Immunol., 157: 11-28, the entire contents of which are hereby incorporated by reference. CMAP is also used for diagnosis of MG patients.
European patent publication no. 432,691 describes an assay for the measurement of direct binding of a peptide that is a T-cell epitope to gene products of the major histocompatibility complex (MHC), classes I and II, on the surface of intact living antigen-presenting cells (APC). The assay comprises incubating the labelled peptide with the APC and monitoring the extent of binding by the addition of a probe that reacts with the ligand used to label the peptide. The assay is suitable for autoimmune diseases and other immunological disorders. With this assay it was demonstrated that p195-212 binds directly to MHC class II molecules on living APC from several different mouse strains. This observed binding capacity for the peptide was shown to correlate with the proliferative potential of the different mouse strains and was inhibited by the relevant anti-I-A antibodies. In addition, it was shown that APC from MG patients and healthy controls, who responded by proliferation to peptides p195-212 and/or p259-271, also bound the same peptides, labelled with biotin. The ability to screen peptides by their direct binding to MHC products and by their stimulatory capacity to T cells might shed light on the role of MG-related epitopes in the pathogenesis of the disease.
It has been suggested that peptide analogs obtained by amino acid substitutions in a stretch of the sequence of a peptidic antigen relevant to an autoimmune disease might lead to peptides that bind to MHC gene products but that do not stimulate specific helper T cells. Such peptides can be used to inhibit competitively T cell reactivity in vitro and in vivo and thus for treatment of the corresponding autoimmune disease (Sakai, K. et al. (1989), PNAS 86:9470; Urban, J. L. et al. (1989), Cell 59:257; European patent publication no. 432,691, and PCT publication no. WO 92/04049). However, none of the references provide any information or guidance on the amino acids of the pathogenic peptide that can be substituted and which amino acids can serve as suitable substituents, in order to obtain a peptide analog that will successfully compete with the pathogenic peptide and thus be useful for preventing or treating autoimmune diseases in general, and myasthenia gravis in particular.